The present invention relates to a variable magnification projecting device for use in electrophotographic copying machines or the like, and more particularly to such a projecting device which comprises, as arranged between an original plane and a projection plane, a projection lens for projecting the original image onto the projection plane and reflecting mirrors disposed in front and rear of the projection lens for folding the optical path of projection, the lens and the mirrors being movable relative to each other so as to project images at varied magnifications with the corrected conjugate length.
U.S. Pat. Nos. 4,571,064 and 3,884,574 disclose variable magnification projecting devices of the type stated above. The first of these conventional projecting devices will be described with reference to FIG. 1. For making a copy, a light source and a first movable mirror b travel leftward in the drawing at a speed of V/M (wherein V is the circumferential speed of a photosensitive drum c, and M is a magnification) to scan a document on a document table d. Simultaneously with this movement, a second movable mirror e and a third movable mirror f travel leftward in the drawing at a speed of V/2M. For varying the magnification, on the other hand, a projection lens g moves on its optical axis, and simultaneously with this movement, a fourth mirror h disposed in the rear of the projection lens g singularly moves on the optical axis for a correction of the length of the optical path.
When varying magnifications with the conjugate length corrected by the movement of the fourth mirror h, the projection lens g is positioned at Lm for a magnification of 1X (same-size magnification), at Ll for the most enlarged magnification, or at Ls for the most reduced magnification, while the fourth mirror h is positioned at Mm for the same-size magnification, at Ml for the most enlarged magnification, or at Ms for the most reduced magnification.
When the fourth mirror h only is moved with the movement of the projection lens g for correcting the conjugate length, the projection lens g and also the fourth mirror h must be moved a large distance in varying the magnification between the most enlarged magnification and the most reduced magnification.
Since the variation of the magnification thus requires a great shift of the optical system, the shift of the optical system which is designed to move at a speed not higher than a specified value to assure precision takes much time, resulting in a prolonged waiting time for the magnification varying movement. In the conventional case, the shift of the projection lens g for giving a varied magnification requires a prolonged period of waiting time.
While the fourth mirror h moves outward from the same-size magnification Mm either for a reduced magnification or for an enlarged magnification, the most reduced magnification position Ms is further outward of the most enlarged magnification position Ml. This influences the external shape or size of the machine body.
In the second conventional device, only mirrors in front of the projection lens move with the lens for a variation in the magnification, and in this respect, the second device differs from the first conventional device wherein only the mirror in the rear of the projection lens moves. However, the two devices are in common in that only the mirror at one of the front and rear sides of the projection lens moves, so that the problem of the first conventional device involved in varying the magnification is also encountered with the second device in varying the magnification.